This invention relates to respirators, for the ventilation of persons whose normal respiratory function is impaired.
Ventilators, or respirators, in medical practice are machines designed to assist a respiratorily impaired patient with breathing. Breathing is a complicated physiological process, but in simplified form, it is a means of delivering oxygen while simultaneously removing carbon dioxide from the tissues of the body.
This is accomplished by the interrelationship of the lungs and by blood circulation; oxygen being transported primarily in bound form in hemoglobin within the blood, and carbon dioxide being primarily transported in solution form in blood plasma.
The most exact indication of proper ventilation is obtained through means of blood gas studies, which can accurately indicate the dissolved and transported oxygen and carbon dioxide within the patient's blood. Since both oxygen and carbon dioxide are ultimately transported across tissue interfaces by diffusion, the partial pressures of the gasses in the solution are the most important criteria, as these indicate whether adequate flow of oxygen and carbon dioxide is occurring.
In a respiratorily impaired patient, a mechanical ventilator is used to augment the breathing process. With current ventilators and modern day medical practice, a number of variables are routinely controlled in order to achieve adequate ventilation of the patient. Primarily controlled variables include the percentage of oxygen in the gas delivered to the respiratory patient's lungs, the number of breaths per minute delivered to the patient, and the tidal volume or total volume of air interchanged in each delivered breath.
Secondary variables which are important in specific patients include positive end expiratory pressure (PEEP), the time of inspiration and expiration with relation to the overall cycle of the breath and the peak flow rate of gas being delivered during a breath. These latter variables may be controlled by a physician as required to overcome certain disease or injury related processes or degradations of the lungs.
When the patient is placed on a ventilator, positive gas passing to the lungs is provided either by an inserted endotracheal tube or, in some cases, by means of a tracheotomy. Depending upon the condition of the patient, various of the above controlled variables are set, the most important being the percentage of oxygen within the respiratory gas to be delivered, the number of breaths per minute to be delivered, the volume of each breath, and the amount of pressure to be maintained in the lungs after expiration (PEEP). After a patient is placed on a ventilator, there are two major goals in his treatment: the first is to decrease the percentage of oxygen in delivered gas to an acceptable partial pressure, as high oxygen percentages over a long period of time are toxic to the lungs; the second is to allow the patient to breathe for himself as much as possible, decreasing the actual mechanical breathing performed by the ventilator.
Various prior art sensors and control combinations have been suggested. Thus, Schultz, U.S. Pat. No. 4,326,513, suggests a control system within a respirator, using a sensor to directly measure the arterial partial pressure of oxygen which, in a closed loop through a mixer, mixes oxygen with other breathing gasses so as to minimize the oxygen concentration while maintaining a desired arterial oxygen partial pressure. This is limited, however, by the lack of a suitably accurate, medically approved, real time sensor for arterial partial pressure of oxygen.
U.S. Pat. No. 3,734,091 to Taplin discloses the use of a device to detect super-saturation of oxygen in the body, together with a control device which produces a temporarily anoxic (oxygen deficient) status within the user to control the oxygen concentration in the breathing mixture. The device is described as being used to maintain close to one hundred percent saturation of oxygen within the body, but is unusable in a respiratorily inhibited patient as the deliberate induction of an anoxic state in such a patient is extremely hazardous.
Separately, a number of patents have used CO.sub.2 analysis of exhaled gas to start and stop respirator action. U.S. Pat. No. 4,537,190 to Caillot discloses the use of a CO.sub.2 analysis cell within the exhaled air together with a control unit which turns the respirator on or off depending upon the level of exhaled CO.sub.2 detected.
U.S. Pat. No. 4,617,924 discloses a particular CO.sub.2 detection and controller of use in a high frequency ventilation system, disclosing an adaptation to permit final expiatory CO.sub.2 concentration to be more accurately determined when the exhaled gas may be intermingled with flush gas, to alleviate an inaccuracy otherwise inherent in high frequency ventilation systems, due to the dilution of the exhaled gasses by flush gas flow.
U.S. Pat. No. 4,016,876 to Martin discloses a breathing apparatus for a healthy user, particularly a fire fighter's breathing system, in which the amount of exhaled carbon dioxide is utilized to trigger replenishment within an air re-breathing apparatus. The device is suitable solely for control of closed cycle respirators for use by otherwise healthy personnel.
A similar, constant volume re-breathing system is shown in U.S. Pat. No. 3,951,137 to Conkle, which utilizes detection of differential pressure, by measuring the inhalation and exhalation pressure induced by respiration of the user to trigger a following effect in the respirator-breathing apparatus.
U.S. Pat. No. 4,612,928 to Tiep discloses a method of minimizing utilization of oxygen within an oxygen re-breather by using a pulse detection circuit to detect breathing cycles in the individual and by supplying oxygen only during inhalation steps.
Sensors for the monitoring of exhaled gasses are shown in U.S. Pat. No. 4,631,966 to Brugnoli and U.S. Pat. No. 4,602,653 to Ruiz-Vela.